1. Field of the Invention
This invention relates to a method of controlling a vehicle driving system adapted to accumulate power output intermittently from a thermal engine in a flywheel as rotational energy and only a necessary part of the accumulated energy is taken out to the output shaft of a power dividing type power transmission unit. The present invention also relates to a vehicle driving apparatus for driving a vehicle driving system by means of such a control method. Thermal engines to which the present invention is applicable include internal-combustion engines and external-combustion engines. Generally, power dividing type power transmission unit comprises a power dividing mechanism, which may be a type using a motor generator where both an inner rotor and an outer rotor rotate or a type where an ordinary motor generator is connected to the reactionary shaft of a differential gear.
2. Description of the Related Art
Known methods of controlling a vehicle driving system of the type under consideration include a control method of accumulating energy in a flywheel and driving a vehicle as disclosed in Jpn. Pat. Appln. Laid-Open Publication No. 2003-020970. The method disclosed in the above cited patent document can be realized by using a drive unit as illustrated in FIG. 1 of the accompanying drawings. Referring to FIG. 1, the drive unit comprises an engine 1, which is an internal-combustion engine, a flywheel 3, a power dividing type power transmission unit 21 and a control unit 27. The control unit 27 typically includes a CPU (central processing unit), a computer equipped with a memory and a power control unit. The signal Acc that is input to the control unit 27 is a signal that corresponds to the pedaled quantity of the accelerator pedal operated by the driver of the vehicle.
The output shaft 1a of the engine 1 is interlocked with the drive shaft 3a of the flywheel 3 by way of a multiplying device (e.g., a speed-up gear) 1A, a drive shaft 1b and a clutch 2. For the purpose of the present invention, the expression of “interlocked” refers to a state where two rotary members are driven to rotate integrally by way of a drive shaft or a state where two rotary members are linked to each other and driven to rotate by way of a power transmission system of a type or another. Examples of mode where two rotary members are linked to each other and driven to rotate by way of a power transmission system of a type or another include a state where they are linked by a gear and a chain and a state where they are linked by a clutch. In FIG. 1, the multiplying device 1A is not indispensable and the output shaft 1a may be directly linked to the drive shaft 1b. 
The power dividing type power transmission unit 21 is realized as an electric type power dividing gear box that includes a first motor generator 4, an output shaft 6 and a second motor generator 5. The first motor generator 4 has an outer rotor 4A, an inner rotor 4B, an input shaft 4a and an outlet shaft 4b, of which the input shaft 4a is interlocked with the outer rotor 4A while the outlet shaft 4b is interlocked with the inner rotor 4B. The outer rotor 4A is interlocked with the flywheel 3 by way of the input shaft 4a. The inner rotor 4B is interlocked with the output shaft 6 by way of the outlet shaft 4b. The output shaft 6 is interlocked with the drive wheels 20, 20 of a vehicle. The pedaled quantity Acc input to the control unit 27 is a signal that operates as a command given by the driver to the control unit 27 to set the level of the torque of the output shaft 6.
The first motor generator 4 and the second motor generator 5 are connected to each other by way of the control unit 27 so that the electric power generated by the first motor generator 4 can be transmitted to the second motor generator 5 by way of the control unit 27. The second motor generator 5 is interlocked with the output shaft 6 by way of a drive shaft 5a and gears 5b and 5c. Note, however, that the second motor generator 5 may alternatively be linked directly to the output shaft 6 without using the gears 5b and 5c. 
Referring to FIG. 1, when the engine 1 is driven to operate and the clutch 2 is held in an engaged condition, the output torque Te of the engine 1 is converted to torque Te/i at the drive shaft 1b, the clutch 2 and the drive shaft 3a due to the existence of the speedup ratio i of the multiplying device 1A. In other words, when the engine 1 is driven to operate, the torque Te/i of the drive shaft 3a drives the flywheel 3 to accelerate the rotational motion of the latter and causes the first motor generator 4 to operate as generator. On the other hand, when the engine 1 is not driven to operate, the first motor generator 4 is driven to operate as generator only by the rotational energy of the flywheel 3.
When the first motor generator 4 is operating as generator, torque T1i is produced at the input shaft 4a of the motor generator 4 and reaction torque T1i having a magnitude same as the toque of the input shaft 4a is produced at the outlet shaft 4b regardless if the engine 1 is in operation or not. The torque T1i produced at the outlet shaft 4b mechanically straightly drives the output shaft 6.
In principle, all the electric power generated by the first motor generator 4 is supplied to the second motor generator 5 and the second motor generator 5 is driven to operate as motor by the electric power so that the mechanical power produced by the second motor generator 5 that is operating as motor by turn drives the output shaft 6 by way of the gears 5b and 5c. The mode of control in which “all” the electric power generated by the first motor generator 4 is supplied to the second motor generator 5 is referred to as “basic control” hereinafter.
Of the mechanism illustrated in FIG. 1, the part of the power transmission system 21 from the input shaft 4a to the output shaft 6 is adapted to operate as power dividing type power transmission unit where the power output from the engine 1 is once divided into powers of different states of energy by the first motor generator 4 (a mechanical state and an electric state in the instance of FIG. 1) and the mechanical power obtained at the outlet shaft 4b by the division and the electric power are put together at the output shaft 6. Such a power dividing type power transmission unit is already known.
In the flywheel energy accumulation/driving system of FIG. 1 having the above described configuration, the relationship of formula (a) shown below needs to be established for the engine 1 to operate to generate torque Te/i at the drive shaft 3a and drive the flywheel 3 to rotate and accelerate the rotational speed thereof:Tf=(Te/i)−T1i>0  (a),where Tf is the acceleration torque of the flywheel 3.
Additionally, the relationship of formula (b) shown below is required for the torque T1i of the input shaft 4a in the formula (a) above:T1i=T2/[1+{(Nf/N2)−1}×ηe]  (b),where T2 is the torque indicated to the output shaft 6 and Nf is the rotational speed of the flywheel 3 while N2 is the rotational speed of the output shaft 6 and ηe is the power transmission efficiency that is observed when the mechanical input power for driving the motor generator 4 to generate electricity is converted into electric power once in the motor generator 4 and turned back to mechanical power by the motor generator 5, which is then transmitted to the output shaft 6.
The above formulas (a) and (b) are stored in the memory of the control unit 27 and used for arithmetic operations by the CPU whenever necessary. The control unit 27 operates for the above-defined basic control according to the outcome of the arithmetic operations. More specifically, the efficiency ηe is experimentally determined in advance and the rotational speed Nf of the flywheel 3 and the rotational speed N2 of the output shaft 6 are detected in advance while the vehicle is running. Then, as the torque T2 of the output shaft 6 is indicated by means of the accelerator pedal, the, value of the input torque T1i of the input shaft 4a for causing the motor generator 4 to generate electricity is determined from the formula (b) and then used to substitute the input torque T1i of the formula (a). Then, the engine 1 is operated to meet the requirement of the formula (a). Since Te=0 when the engine 1 is not operated, the flywheel torque Tf in the formula (a) is Tf<0. The rotational speed Nf of the flywheel 3 is being decelerated in such a situation.
When the driver of the vehicle controls the moving speed of the vehicle by operating the accelerator pedal, he or she is only required to indicate the torque T2 of the output shaft 6 regardless if the engine 1 is in operation or not. Then, the control unit 27 sets the power generating torque of the motor generator 4 to T1i according to the indication of the torque T2, using the formula (b).
When the rotational energy accumulated in the flywheel 3 becomes insufficient as a result of that the output shaft 6 is driven to rotate only by the rotational energy of the flywheel 3 without operating the engine 1 with the above described basic control, the clutch 2 is then linked to accumulate rotational energy in the flywheel 3 by the engine 1 according to the above formula (a), while continuing the operation of driving the output shaft 6,